This invention relates to an exhaust manifold for internal combustion engines and, more particularly, to an exhaust manifold in which exhaust ducts for guiding exhaust gases to the exhaust pipe or exhaust pipes are formed of discrete pipe members.
Exhaust manifolds for internal combustion engines are comprised of a plurality of exhaust ducts having exhaust conduits defined therein and connected to respective cylinders of the engine, and at least one united section in which exhaust gases flowing through the exhaust ducts are gathered together. Conventional exhaust manifolds of this kind are generally formed of castings such as iron casting in which the exhaust ducts and the united section are integrally formed in a one-piece body. Also flanges provided at upstream ends of the respective exhaust ducts for connection with the cylinder head as well as a flange provided at a downstream end of the united section for connection with the exhaust pipe or exhaust pipes are formed integrally with the exhaust ducts and the united section, respectively.
Problems encountered with such conventional exhaust manifolds formed of castings are that it is difficult to reduce the weight, and it is also difficult to obtain desired smoothness of inner wall surfaces of the exhaust conduits and equality in cross-sectional area between the exhaust ducts, involving the disadvantage that exhaust gases cannot be smoothly discharged through the exhaust manifold. In order to solve the problems with the conventional exhaust manifolds made of castings, exhaust manifolds of a so-called pipe structure have lately been employed in which the exhaust ducts are formed of discrete pipe members.
A typical example of the conventional exhaust manifolds made of discrete pipe members is shown in FIG. 1, wherein the exhaust manifold comprises four exhaust ducts 1-4 formed of discrete pipe members, united tubes 5, 5' constituting a united section connected with respective pairs of the exhaust ducts 1-4 at downstream ends thereof, a flange member 6 formed with a plurality of integral flanges for connection with a cylinder head, not shown, and a flange 7 for connection with the united tubes 5, 5', all the component parts being fabricated in separate bodies from each other and joined together by means of welding or the like. The united tubes 5, 5' are generally formed of sheet metal or a like material and have their ends fitted on downstream ends of the exhaust ducts 1-4 connected to respective engine cylinders, not shown, and welded thereto. The flanges 6, 7 are formed of castings or forgings.
The illustrated exhaust manifold is adapted for use in a four-cylinder internal combustion engine, and is of a so-called "dual exhaust manifold structure" which is so configurated that each of the united tubes 5, 5' is to be connected with engine cylinders which are so located relative to each other that the exhaust stroke of one of the cylinders does not take place immediately after that of the other cylinder, so as to avoid interference of exhaust discharge with each other.
Exhaust manifolds, if used in electronically controlled internal combustion engines in which the air-fuel ratio of a mixture supplied to the engine is electronically controlled in response to operating conditions of the engine, etc. have to be adapted for installation of an exhaust constituent-concentration sensor such as an O.sub.2 sensor therein for sensing the concentration of an exhaust constituent such as oxygen in the exhaust gases emitted from the engine to detect the actual air-fuel ratio of the mixture.
However, the aforesaid conventional exhaust manifolds of the pipe structure are not structurally suited for the installation of O.sub.2 sensor therein. That is, in the case of providing O.sub.2 sensors respectively in the exhaust ducts of an exhaust manifold of this type it is necessary to form a bore in each exhaust duct through which a sensing probe of an O.sub.2 sensor is to be inserted and also to weld a separately fabricated boss plate formed therein with a sensor-fitting hole to a portion of the exhaust duct formed with the above bore. In the case of providing O.sub.2 sensors in the united section, although so many O.sub.2 sensors are not required to be used as in the former case, the O.sub.2 sensors will be located at lower temperature downstream portions of the manifold which are unfavorable to activation of the sensors, and it is necessary to form a bore in the united section through which a sensing probe of an O.sub.2 sensor is to be inserted and also to weld a separately fabricated boss plate formed therein with a sensor-fitting hole to a portion of the exhaust duct formed with the above bore, as in the former case. Therefore, in either case, the number of component parts is necessarily large, the fitting operation of the O.sub.2 sensors is difficult and complicated, and in the dual exhaust manifold in particular, in which all the united tubes for all the cylinders are located at a downstream side of the manifold, there is a limitation in the location at which the sensors are to be mounted in the manifold.